This invention relates to the reduction of sound radiated from vibrating surfaces.
The prevention or attenuation of sound radiating from noisy equipment is a continuing problem. There are many techniques known in the prior art, each having its own merits and limitations. Some of the known techniques and their limitations are described below.
Barriers
The mechanical impedance of a barrier is the ratio of an applied force to the resulting vibration velocity. For a given applied force, a higher mechanical impedance will result in a lower vibration velocity, and hence a lower level of radiated sound. A sound barrier is therefore designed to have a high mechanical impedance. In traditional sound barriers this is achieved by using structures with high mass and/or high stiffness. The concrete walls alongside highways, which are both massive and stiff, are an example of this kind of barrier. The barriers must be relatively tall because diffraction, thermal shear and wind shear allow the sound to leak around the barrier. When the noise source is stationary, an alternative is to put the barrier close to the noise source, but this is often impractical because access may be required or because the presence of the barrier prevents heat loss and may cause the machine to overheat. When the barrier completely contains the noise source it is referred to as an enclosure. A light weight acoustic enclosure is described in U.S. Pat. No. 5,804,775 (Pinnington), for example.
An alternative method for obtaining a barrier, which has a high impedance at specified discrete frequencies, is described in U.S. Pat. No. 4,373,608 (Holmes). This uses mechanical resonators distributed over the surface of a sound barrier to provide a high impedance at the resonance frequency.
A still further approach, disclosed in U.S. Pat. No. 4,600,078 (Wirt), uses acoustic resonators inside a double-leaf barrier to increase the compliance of the enclosed volume.
Vibration Control
Vibration control seeks to control the vibration of the noise source directly. For a vibrating machine, this is done by increasing the mechanical impedance of the machine structure. One way to do this is by adding mass and/or stiffness to the vibrating structure.
A further method is to use mechanical resonators, as also described in U.S. Pat. No. 4,373,608 (Holmes). The resonators can be attached directly to the surface of a vibrating machine. An example of this type of control is a tuned dynamic absorber. These have been used successfully to reduce noise inside aircraft.
A still further method into use an active vibration control system. Examples include U.S. Pat. No. 4,435,751 (Hori), U.S. Pat. No. 4,525,791 (Hagiwara et al), U.S. Pat. No. 4,715,559 (Fuller) and U.S. Pat. No. 5,519,637 (Mathur). This method uses force actuators to apply forces to the vibrating surface, and thereby increase its apparent mechanical impedance.
In practice, many machines are already very high impedance structures excited by large forces. Often it is not possible to obtain much change in the combined impedance. Consequently it is difficult to reduce effectively the vibration and resulting sound radiation.
Active vibration control may also be attempted by using piezo-electric patches applied to the surface of the vibrating structure. These can be used to control bending of the structure, but do not prevent sound radiation by planar motion of a surface.
Further disadvantages of this method include the need for acoustic sensors to monitor the performance of the system and the need for a power supply. These add to the cost and complexity of the system.
Vibration Isolation
The simplest example of vibration isolation is a resilient machinery mount. When the frequency of the source of vibration (e.g. the rate of rotation of a motor) is significantly above the resonance frequency of the machine itself on its mounts, the foundation is isolated from the vibration of the machine. Another example is a double-leaf partition wall, which comprises two relatively high impedance panels separated by a low impedance intermediate layer (which is often air). Above the resonance frequency, the inertia of the radiating panel is much higher than the force required to compress the intermediate layer, so little vibration is transmitted to the radiating panel.
A further approach, disclosed in U.S. Pat. No. 5,315,661 (Gossman et al.), uses active control to isolate the outer leaf of a panel.
A further example is provided by U.S. Pat. No. 4,442,647 (Olsen). This uses a resonant device to reduce the radiation from a fuselage wall into a helicopter cabin.
Vibration isolation is often unsuitable for reducing the sound radiated from vibrating machinery, since it is often impractical to completely enclose the machinery because of access and cooling requirements.
Modification of Acoustic Impedance
Devices which have a low impedance (relative to the fluid medium into which the sound radiates) can be used to modify the acoustic impedance and thereby alter the sound field. Examples include Helmholtz resonators and mechanical resonators. U.S. Pat. No. 4,149,612 (Bschorr) and an associated paper xe2x80x98The Silatorxe2x80x94A Small Volume Resonatorxe2x80x99, O. Bschorr and E. Laudien, Journal of Sound and Vibration (1992), 158(1), 81-92, describe such a resonator. These are effective for controlling sound in a waveguide, where an impedance change can cause a reflection. However, they are of limited effectiveness in stopping radiated sound. Since the resonator is driven by the acoustic field, the sound cannot be cancelled, as there would then be nothing to drive the resonator. Instead, the resonator moves in quadrature (at 90xc2x0 phase angle) to the acoustic field. Table 2 of the paper by O. Bschorr and E. Laudien indicates that the noise reduction is limited to 6 dB for wall emissions.
Active Sound Control
It is well known that the noise from a radiating surface can be reduced by placing secondary sources on or around the surface. See for example xe2x80x98The Active Control of Transformer Noisexe2x80x99, G. P. Eatwell, Proc. Inst. Acoust., 9(7), 1987, p269 and xe2x80x98Secondary Sources and their Energy Transferxe2x80x99, M. J. M. Jessel, Acoustics Letters, Vol. 4, No. 9, 1981.
Active sound control uses computer controlled acoustic sources close to the primary noise source. The amplitude and phase of the sources is chosen so that the farfield radiated noise is reduced. Since the radiation pattern of the vibrating surface is seldom fixed, active control systems require acoustic sensors in the farfield to monitor performance and adjust the amplitude and phase of the controlled sources. This requirement adds significantly to the cost and complexity of the system and limits this technology to applications in which the noise source is acoustically compact or where very large costs can be borne. In addition, the complexity of the system necessitates regular maintenance, which further adds to the cost. Also, an active control system requires a power source, which complicates the installation process and is impractical in some applications. These features make active control systems expensive when compared to passive noise control methods.
There are many examples of this approach, including U.S. Pat. No. 4,025,724 (Davidson et al.) and U.S. Pat. No. 5,381,381 (Sartori et al.) which use near field acoustic sensors to provide reference signals, and U.S. Pat. No. 4,930,113 (Sallas), U.S. Pat. No. 5,245,664 (Kinoshite et al.), U.S. Pat. No. 5,410,607 (Mason) and U.S. Pat. No. 5,642,445 (Bucaro et al.) which use vibration sensors to provide reference signals.
Object of the Invention
Therefore, there is a need for a passive sound reduction system which (i) has low cost and high reliability (ii) can be applied to structures which have very high mechanical impedance (iii) allows for cooling and access to the structure and (iv) is easy to install. None of the methods of the prior art combines these properties,and it is accordingly an object of the invention to do so.
The vibration excited sound absorber of the current invention provides a method and apparatus for reducing the sound radiated from a vibrating surface into a surrounding fluid. The fluid may be liquid or gas. The apparatus has low cost and high reliability and can be applied to any structure, including structures which have a very high mechanical impedance. When applied directly to the surface of a machine, the apparatus only partially covers the structure and so allows for cooling and access. Multiple sound absorbers can be applied to any vibrating surface, including walls and existing barriers. The sound absorbers can also be incorporated in custom barriers. Unlike active noise control systems, no special skills are required to determine the positions for the sound absorbers. In one embodiment, the sound absorbers are simply attached to the vibrating surface, so the system can be easily retrofitted to operating equipment.
Examples of applications include power transformers, acoustic enclosures, acoustic barriers, aircraft fuselages etc.
The sound absorber has a radiating element and a coupling element which together have a tuned dynamic response. The coupling element couples the motion of the radiating element to that of the vibrating surface. The radiating element is thereby excited into motion by the vibration of the surface. The vibrating surface is partially covered with one or more sound absorbers. The dynamic response of the sound absorber is tuned so that acoustic volume velocity of the radiating element is substantially equal in amplitude but opposite in phase relative to the volume velocity of the surrounding exposed vibrating surface. The net volume velocity of the surface is thereby reduced. For example, if radiating elements cover 10% of the vibrating surface, preventing the covered portion from radiating sound, each radiating element must have a velocity nine times that of the vibrating surface, but in the opposite direction. The volume velocity of the radiating element then cancels the volume velocity of the remaining 90% of the vibrating surface. This is in contrast to vibration isolation, in which the aim is to make the volume velocity of the sound absorber as small as possible. Vibration isolation is only effective when the entire vibrating surface is covered.
The radiating element can be solid or fluid, and is coupled to the vibrating surface by a coupling element.